This invention relates generally to a device for collecting and filtering foreign matter and debris from drilling mud flowing through a drill string. More particularly, this invention relates to a debris catcher and filter positioned downhole within the drill string upstream of a mud powered turbine (or other downhole device, the operation of which might be jeopardized by debris in the mud stream) to collect and filter miscellaneous debris from the drilling mud prior to its entering the turbine, while still permitting large quantities of lost circulation material to pass therethrough.
A well-known problem in the oil well drilling and related fields has been the undesirable presence of foreign matter and debris in the drilling mud which flows through a drill string. Such debris will act to jam or disrupt instrumentation and other devices in the drill string. Surface operated devices, such as desanders or filters, are known for filtering the mud at the surface. However, these surface devices do not solve the problem of debris which enters the mud downstream from the surface equipment, such as, for example, debris which breaks loose from the inner surface of the pipe.
Measurement-while-drilling (MWD) system which use mud powered turbines to generate power downhole are face with a particularly serious problem from foreign matter in drilling mud. It is well-known that such mud powered turbines are subject to field failure when debris in the drilling mud lodges in the turbine to jam or prevent the turbine rotor from rotating. While prior art filters or debris catchers are known, in many cases the drilling operator resists installing such known filters or debris catchers in the drill string because of a concern that the filter may become clogged if lost circulation material is pumped downhole. A clogged filter acts to halt circulation of drilling mud and leads to even more serious problems than those problems discussed above with regard to the existance of debris in the drilling mud. In other words, an operator would rather have debris in the drilling mud than take a chance that by using a filter, the filter would become clogged and hence, the drilling mud would cease to circulate.
Because of this fear of clogged debris catchers or filters, prior art devices (fishing plugs) have been provided for retrieving a part of the filtering apparatus should it become clogged. In that case, by retrieving the filtering apparatus, an opening will be provided for the passage of mud. However, even those debris catchers which provide means for retrieving a part of the filtering apparatus are unacceptable to many MWD users because of a concern that operating circumstances may make it impractical to retrieve a fishing plug and thereby open a flow area through the debris catcher.